Wave soldering is a technique for joining electronic devices or pretinning metallic components. It is accomplished by bringing the components in contact with at least one standing wave of molten solder. The process is described in "Soldering in Electronics", by R. J. Wassink, Electrochemical Publications Ltd., Ayr, Scotland, 1984, pages 332 to 361, which is incorporated by reference herein.
The process typically, but not necessarily, includes the application of a flux to the surfaces to be wet by the solder. The flux treats the surfaces so that the solder will adhere to them. The flux covered surfaces are commonly heated prior to soldering in order to dry and activate the flux and reduce thermal shock.
A solder wave is formed by pumping molten solder, contained within a solder pot, up through a nozzle to provide a standing wave. Usually, only one wave is formed, but dual waves are also employed, particularly when surface mounted devices are being soldered to printed circuit boards. Solder cascades and solder jets find application in wave soldering also.
One of the problems encountered with wave soldering is that the molten solder oxidizes when exposed to oxygen. The oxidized solder forms a surface oxide layer, which must be removed by a flux before the components being soldered will wet. The surface layer is continually broken by the flow of solder in the wave. This exposes fresh solder which, in turn, is also oxidized. A mixture of oxide and solder, thus, collects within the solder pot. This mixture is known as dross, which must be removed and disposed of. Dross generation adds to the cost of the process due to the lost value of the solder and the maintenance time required to remove it and repair mechanical parts of the wave soldering apparatus damaged by the abrasive action of the dross. Dross can also be a health hazard when the metals or oxides in it are toxic, e.g., lead or lead oxide. The hazard may surface when workers handle the dross in removing it from the solder pot or when removal of the dross results in the suspension of dross particles in ambient air and their subsequent inhalation.
One method employed to minimize the formation of oxide on the solder wave is to cover the surface with an oil. This is effective in protecting the solder from atmospheric oxygen, but the oil degrades and must be replaced periodically. Furthermore, the oils commonly used are difficult to clean off of the components being soldered and can produce a great deal of smoke at wave soldering temperatures.
A more satisfactory technique, which has been suggested for the reduction of dross formation, is blanketing the entire surface with an inert atmosphere, i.e., replacing the air or oxygen residing on and above the surface of the wave with an inert gas. Unfortunately, this results in an increased incidence of a phenomenon known as bridge formation. "Bridges" are solder joints formed between components or parts of components, which are not intended to be joined. In electronic devices, a bridge leads to a short circuit.